I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for equalization in a speech communication system.
II. Description of the Related Art
Transmission of voice by digital techniques has become widespread, particularly in long distance and digital radio telephone applications. This in turn has created interest in determining methods which minimize the amount of information sent over the transmission channel while maintaining high quality in the reconstructed speech. If speech is transmitted by simply sampling and digitizing, a data rate on the order of 64 kilobits per second (kbps) is required to achieve a speech quality of conventional analog telephone. However, through the use of speech analysis, followed by the appropriate coding, transmission, and resynthesis at the receiver, a significant reduction in the data rate can be achieved.
Devices which employ techniques to compress voiced speech by extracting parameters that relate to a model of human speech generation are typically called vocoders. Such devices are composed of an encoder, which analyzes the incoming speech to extract the relevant parameters, and a decoder, which resynthesizes the speech using the parameters which it receives over the transmission channel. The model is constantly changing to accurately model the time varying speech signal. Thus, the speech is divided into blocks of time, or analysis frames, during which the parameters are calculated. The parameters are then updated for each new frame.
Of the various classes of speech coders, the Code Excited Linear Predictive Coding (CELP), Stochastic Coding, or Vector Excited Speech Coding coders are of one class. An example of a coding algorithm of this particular class is described in the paper "A 4.8 kbps Code Excited Linear Predictive Coder" by Thomas E. Tremain et al., Proceedings of the Mobile Satellite Conference, 1988. Similarly, examples of other vocoders of this type are detailed in U.S. Pat. No. 5,414,796, entitled "Variable Rate Vocoder", which is assigned to the assignee of the present invention and incorporated by reference herein.
In the transmission of speech signals, the perceptual quality is of primary importance to users and service providers. Extensive studies have been conducted to determine what the most perceptually pleasing spectral response is to listeners. In response to these studies, systems have been developed that uniformly boost the bass response and reduce down the high end response of the speaker. The usefulness of such systems, however, is premised on a uniform input source. In systems where there is variety of possible input sources each with a unique spectral response characteristic, there is a need for spectral equalization that takes into account the effects of different input sources.